The present invention relates to the field of electric batteries designed for use with electric motors which are rechargeable using regenerative charging, such as batteries for electric bicycles. More specifically, the present invention relates to a reconfigurable battery, reconfigurable electric motors for use with such a reconfigurable battery, methods for reconfiguring a battery for driving variable electrical loads, and methods for reconfiguring a battery for charging and for reconfiguring electric motors for charging a battery.
The present invention is described in connection with electric bicycles where a rechargeable battery drives an electric motor. In prior art electric bicycles, in some instances the current from the battery is regulated by a speed controller that controls the motor which provides assistance to the rider. In other instances, where the rider wants to slow down or brake going downhill, the motor acts as a generator and supplies the current back to the battery, thereby achieving regenerative braking that recovers part of the energy that would otherwise be lost when using a mechanical brake alone.
An electric motor typically uses a set of magnets, for example, electro magnets and permanent magnets. As the motor turns, the attractive and repulsive forces of these magnets are regulated electrically such that the motor turns continuously in the desired direction. This could be done by electro-mechanical switches (e.g. commutators), or could be done by solid state switches (e.g. FETs—Field Effects Transistors). FIG. 1 shows an example of a motor 12 connected to a battery 10. As the current Im flows into the motor 12 and the motor turns, the motor generates a back EMF (Electro Motive Force) which is a voltage roughly proportional to the speed of the motor 12. The current Im is defined as (VB−VM)/(RM+RB) where RM is the internal resistance of the motor 12 and RB is the internal resistance of the battery. Given a fixed applied voltage VB (e.g. from the battery 10) the back EMF reduces the amount of current that flows into the motor 12, because the current flow is proportional to the difference between the motor voltage VM (back EMF) and the battery voltage VB. For example, if the motor 12 is turning (with some outside assistance) at a rate such that the back EMF equals the battery voltage VB, than there will be no current flow. If the motor 12 turns faster than this such that the back EMF is higher than the battery voltage VB, then the current flows the other way, thereby recharging the battery 10. One extreme case is a stall, when the motor 12 is at rest. In such a case, the back EMF is zero since the motor is at rest, the current flow from the battery 10 will be at its maximum, and the motor 12 will produce its highest torque.
When the bicycle is moving and the motor 12 produces a finite back EMF, the motor 12 can be used as a generator to recharge the battery 10, while achieving a desired level of braking. In order to achieve this, the voltage out of the motor 12 is increased to a level higher than the battery 10 using a device known as an inverter.
A block diagram of a typical prior art electric bicycle system without regenerative braking is shown in FIG. 2. A battery 10 provides current to a motor 12 though a speed controller 11. The speed controller 11 governs the current flow to the motor 12, thereby controlling its speed. The speed controller 11 may be set to a desired speed by a rider using a control knob 13.
A block diagram of a further prior art electric bicycle system that provides regenerative braking is shown in FIG. 3. FIG. 3 is similar to FIG. 2 but also includes an inverter 14 in parallel with the controller 11. A switch 15 is provided for coupling the motor 12 to the controller 11 (in a drive mode) or the inverter 14 (in a braking mode). During the braking mode, current is generated by the motor 12 and passed to the battery 10 by the inverter 14, in order to charge the battery.
It should be noted that a practical system involves two distinct operations, one that drives the motor and the bicycle wheel(s) by supplying current from the battery to the motor(s), and another that uses the current from the motor(s) to charge the battery to achieve regenerative braking, thereby slowing down the bicycle. It should be further apparent from FIG. 3 that in order to recharge the battery, one needs an inverter that increases the voltage from the motor to a value higher than the battery voltage, in order for the current to flow back into the battery.
For a typical rechargeable battery, the charging voltage must be higher than the battery voltage. The higher the charging voltage relative to the battery voltage, the more current flows into the battery. Controlling the charging voltage is one of the ways to control the rate of recharging, as well as the rate of braking. Another way to control the recharging rate is pulse width modulation (PWM), where a switch between the charging source and the battery regulates an on-off duty cycle. Of course, the charging voltage still needs to be higher than the battery voltage for such a device to work.
In most electric vehicles such as electric bicycles and electric cars that utilize regenerative braking, the electrical system typically consists of several subsystems, namely a motor, a speed controller, an inverter, and a battery. Sometimes the speed controller regulates both the drive and braking current via PWM. Potentially, a clever inverter design could regulate both driving and braking by regulating the voltage to the motor for driving, and regulating the voltage to the battery for regenerative braking, thereby eliminating the need for a separate speed controller.
However, an inverter is not an easy device to design or cheaply produce, as it must handle a large amount of current (especially during quick braking) and sometimes a high output voltage, while its input voltage can fluctuate over a wide range. The input voltage in this case is the back EMF from the motor, typically close to zero when the bicycle is coming to a stop, and close to the maximum battery voltage when the bicycle is coasting on a level ground at its maximum speed (usually the battery voltage limits the top speed).
Also an inverter typically achieves its functionality using rapid switching devices. One inverter design could turn the DC current from the motor to AC current first, increase the voltage using a step-up transformer, and convert the AC current back to DC in order to recharge the battery. Another inverter design could use temporary energy storage elements such as capacitors and inductors in a charge-pump configuration in order to raise the voltage. The switching frequency involved is typically in the order of 1-100 KHz. In most of the known inverter designs, the energy loss is significant, and the cost is very high due to the high current requirement (100 Amps or more) in addition to the weight. For this reason, only a small percentage of electric bicycle products incorporate regenerative braking in their design.
It would be advantageous to provide a battery and/or electric motor configuration that provides driving and regenerative braking, for example in an electric bicycle, over a reasonable range of operations without the need for an inverter.
It would also be useful to provide a reconfigurable battery and battery control system that provides duty cycle modulation of an array of battery cells for intermediate output voltage control without incurring large switching losses, while simultaneously reducing switching induced transient signals.
The methods and apparatus of the present invention provide a series connected reconfigurable battery having these and other advantages.